Theres' a few sites likes this. Personally I would avoid as you have to pay for each bid. Clearly that's how they make their money, but whats the chances of you getting something so cheaply. Swoopo.co.uk for instance charge 50p per bid and have a countdown clock. Each time a new bid is made the clock resets, so you can imagine how many 50p's you're gonna need.

don't use reverse auction sites myself, but my brother's friend did recently and got a huge bargain.
He wanted a laptop(worth around £1000), he was gonna buy anyway, this site offered the option to buy outright for around £1200 (all your bids are refunded/cancelled if you buy instantly). In the end the price went so high and with all his bids he was better of buying it for £1200.
Now the good part, his purchase also came with something like 100 free bids, so he used them on another laptop(same type), and he got it for less than £100!!!